Component and method for fabricating superconducting wire

ABSTRACT

A component 10 for making Al5 Nb 3  Sn superconducting wire is of plane-filling cross-section after removing temporary additions 6, 7. It consists of a central pillar 1 of aluminium (later replaced by tin) surrounded by a two-deep array of polygonal copper columns 2/2a containing niobium rods. Many (e.g. 61) components 10 are stacked together and extruded. The niobium rods adopt and retain a uniform distribution with minimum intervening material. On heat-treatment of the whole, the tin diffuses over a relatively short path and hence consistently into the rods, whereby there is formed a kilofilament Nb 3  Sn wire.

This invention relates to a component for use in fabricatingsuperconducting wire and to a method of fabricating it, as well as to anintermediate member and to the product.

The theoretical potential of Al5 superconductors such as Nb₃ Sn has beenknown since 1960, but due mainly to their brittleness, in thirty yearsno ideal way of mass-producing them into wires has been found.Contributing to the difficulty is the requirement for the wires toinclude a continuous phase of pure copper, to act as a normal electricalconductor, heat sink and mechanical support in case the Nb₃ Sn isaccidentally warmed above its superconducting range.

The superconducting component should not be thick normal to thecurrent-carrying direction, i.e. It should be merely a filament,otherwise magnetic fields will set up wasteful eddy currents in thecomponent. At the same time, a mere filament would be able to carry onlya small current, and therefore a superconducting wire conventionallyconsists of many parallel non-touching filaments of superconductorembedded in a matrix which is conveniently an ohmic conductor such asbronze or copper. Most conventional ways of mass-producing asuperconducting wire rely on forming some precursor of thesuperconductor to the final required shape, then converting theprecursor. For example, in the so called bronze route, rods of pureniobium are drawn down in a tin bronze to the extent that a fine wire isproduced with filaments of niobium embedded in it. This precursor isthen heated such that the niobium filaments are largely converted toniobium tin by reaction with the tin in the bronze. The maindisadvantage of this route is that if there is more than 13% tin in thebronze it becomes progressively brittle during drawing until it finallybreaks. This means that the mean current density in the final conductoris much reduced by the large volume of bronze required.

The so called internal tin route attempts to avoid the requirement forthis large volume of bronze by including the tin separately in theprecursor in the form of rods which are usually more than two orders ofmagnitude larger than the niobium filaments. There can be problems withdrawing down even such precursors (the tin melts) and, as disclosed inUK Patent Application GB 2201830A, this can be partly mitigated by usingaluminium in place of tin; at a later stage in the method, when thecross-section of the composite has been substantially reduced by drawingdown or extrusion, the aluminium is removed from the composite andreplaced with tin.

The composite (conventionally of circular cross-section) is extruded ordrawn down, then an array of extruded composites is bundled together andfurther extruded, and so on, with as many of these stages as necessary.To avoid rupture of niobium filaments during the first extrusion, arelatively stout outer layer of copper is often left around thefilaments. It will be seen that this leads to opposing designconsiderations. At each of these stages of bundling, these stout layersof copper or subsequent copper extrusion cans become part of the volumeof the final conductor. Since copper competes with niobium for tin, thiswastefully increases the amount of tin which must be provided and alsoincreases the volume proportion of non-superconducting material. Tominimise this effect, the number of extrusion/bundling stages can bereduced. This entails cramming many niobium filaments into each singlestarting composite while, for reasons of manufacturing practicability,the tin remains present in one rather thick rod per composite. On heattreatment to react the niobium filaments with the tin, exchange ofcopper and tin between the regions thereof occurs via relatively longtortuous diffusion paths through the stack of filaments and predisposestowards the formation of Kirkendahl voids caused by the different ratesof diffusion of copper and tin. This is self-evidently a waste ofpotential current-carrying volume. Any design is a compromise betweenthese effects.

The earlier-mentioned pure copper which is required is provided byenclosing arrays of composites inside barrier material such as tantalum,of thickness adequate to retain its integrity through extrusion, andencasing the whole in pure copper.

According to the present invention, there is provided a cylindricalcomponent for use in fabricating superconducting wire, comprising acentral pillar of a stanniferous, galliferous and/or germaniferousmaterial or of an extrudable removable precursor thereof, or ofaluminiferous material, preferably tin, surrounded by a two-deep arrayof cupriferous columns each containing a niobiferous rod, at least theouter set of said columns being polygonal, and the cross-section of thecomponent being a plane-filling shape, whether before or afterextrusion. By "cylindrical" it is clear we mean the word in itstopological sense, not the layman's sense of "right-circular cylinder",since the cylindrical component according to the invention has apolygonal exterior. An advantage of the two-deep array is the shorteningof the gallium/germanium/tin diffusion pathway from the central pillarto its most distant rod compared with GB 2201830 A, thus yielding a moreuniform tin concentration in the product. The niobiferous metal maycontain for example titanium and/or tantalum additives, which increasethe upper critical field of Nb--Sn, e.g. Ti and/or Ta in quantities ofup to 10% by weight.

In the two-deep array, there may be twelve cupriferous columns in theinner set and eighteen cupriferous columns in the outer set, the lattereighteen preferably being regular-hexagonal in cross-section, and atleast some of the inner set preferably being identical. Alternatecupriferous columns of the inner set may be pentagonal in cross-section.

The central pillar is preferably duodecagonal in cross-section.

Such a geometry can more closely approach the optimum proportions of tin(in the central pillar) to niobium with minimum copper while offeringshorter average diffusion paths for the tin than heretofore.

As the cross-section of the component is a plane-filling shape, i.e.,repeated indefinitely in the same size, an unlimited number of thecomponents can be close-packed to fill a plane without voids. (Regularhexagons and squares are examples of plane-filling shapes, but aplane-filling component according to the invention would normally bemore complex in shape.)

The component is preferably further surrounded by removable fillerstrips of an extrudable metal or alloy with a higher melting point thanany one of tin, germanium or gallium, so profiled as to impart to saidcomponent a void-free extrudable cross-section, such as regular hexagonor a circle. Since presses capable of (the theoretically more ideal)hexagonal-to-hexagonal hydrostatic extrusion number well under1/continent, it is alternatively possible to make the componenttemporarily right-circular-cylindrical (using the removable fillerstrips) to widen the choice of extrusion sub-contractors, the fillerstrips and any surrounding extrusion can being removed after theextrusion.

The invention extends to an intermediate member comprising aclose-packed array of the components set forth above (any of saidremovable filler strips having been removed). Because the saidcomponents are not surrounded by the previously necessary stout outerlayer of copper, not only is tin saved and the volume more efficientlyused for carrying current, but the spacing of the said columns in theintermediate member is substantially constant even across the joinbetween adjacent components, thus assisting uniformity of propertiesafter heat-treatment (described later), reducing the risk of Kirkendahlvoids and reducing the risk that when the niobium rods are expanded byabsorbing tin, neighbouring superconductor rods from neighbouring arrayswill come into contact, permitting wasteful eddy currents laterally tothe length of the rods.

The invention further extends to a method of fabricating asuperconducting wire, comprising applying external filler strips to thesaid intermediate member, these strips being so profiled as to impart tothe member a substantially void-free extrudable cross-section, whichitself is preferably plane-filling, such as a regular hexagon, and maybe surrounded by a diffusion barrier such as tantalum foil, optionallywith an exterior niobium layer. The member (preferably then encased inan extrusion can) may then be worked (e.g. extruded or drawn) into theshape of a wire.

At some stage in the above, the central pillars may be removed (e.g.melted or dissolved out, for example if of aluminium, dissolved out byhot sodium hydroxide) and replaced by stanniferous metal, or aluminiummay be left. Then the member may be heat-treated to diffuse the tin oraluminium via the columns into the rods to form the Nb₃ Sn or Nb₃ Alsuperconductor.

The invention therefore extends to a superconducting wire made as setforth above, and to one which has at least one, preferably at least two,such as at least three gallium/germanium cores per hundredsuperconducting filaments, preferably with under 0.1% void volume, andpreferably with a ratio of (actual filament diameter):(effectivefilament diameter) of at least 1/2, such as at least 3/4, such as atleast 0.9.

The invention will now be described by way of example with reference tothe accompanying drawings, in which

FIG. 1 is a cross-section of a cylindrical component according to theinvention, roughly full-size,

FIG. 1A is a cross-section of an alternative design to FIG. 1,

FIG. 2 is a cross-section of an intermediate member according to theinvention, also roughly full-size,

FIG. 3 is a cross-section of an assembly of several of the FIG. 2intermediate members, and

FIG. 4 is a cross-section of a special intermediate, shown enlargedabout tenfold (linear magnification), used in the preparation ofexternal filler strips for the intermediate member.

Turning to FIGS. 1 and 1A (which are alternatives), a cylindricalcomponent 10 comprises a central duodecagonal pillar 1 of aluminiumsurrounded by a two-deep array of polygonal copper columns 2 eachcontaining a cylinder of niobium. Of the thirty columns 2, twentyfourwill be seen to be regular hexagons, the rest 2a being of a specificpentagonal shape (nearly as easy to make both as regards copper andniobium) to fill the shape. (It could be envisaged for the six 2a andthe six others on the inner ring to have an arcuate inner edge, toencircle a circular cross-sectional pillar 1. Other variations are alsopossible. However, the layout in the Figures represents an optimalvolume ratio of aluminium to niobium.) Temporarily, the component 10 issurrounded by aluminium filler strips 6 encased in a strippable copperprotective sheath 7, the whole being substantially void-free and readilyextrudable. The whole is preheated to 200° C. to promote bonding of thestructure.

The whole is extruded to one-thirtieth of the starting cross-sectionalarea, maintaining the hexagonal (FIG. 1) or circular (FIG. 1A)cross-section, whereby internal compression is isotropic and the shape(despite the thirtyfold reduction) is not disturbed at all. Much work isdone, and hence heat is generated, during this operation, and thetemperature rises to a level which would have melted tin but does notmelt the aluminium. The heat usefully bonds the copper columns 2together.

The copper sheath 7 is stripped off and the aluminium filler strips 6are removed by dissolution in caustic soda. The (reduced) component 10is assembled in close-packed (void-free) array with sixty more in agenerally hexagonal array to form an Intermediate Member, indicated as"20" in FIG. 2.

There are now several choices of route to the desired superonductorwire. Five examples will be described.

ROUTE 1

For this route, it may be convenient to go directly to a largerhexagonal array of components 10, the next larger size containingninety-one of them, and the next size again containing 127.

The Intermediate Member (the array of sixty-one (or 91 or 127)components 10) is then surrounded (in the "sixty-one" version as shownin FIG. 2) by twentyfour filler strips 21 and six corner filler strips22 to present a regular hexagonal exterior. This is wrapped in tantalumfoil 23, which acts as a tin diffusion barrier. In this and thealternative Routes, the Intermediate Member wrapped in tantalum foil 23may then be wrapped in niobium foil, not shown. (The filler strips 21and 22 are described in more detail later.)

Let this be Stage A. Then arcuate filler strips of copper are appliedaround the tantalum foil 23 (or of course the niobium foil if present),the copper strips being so profiled as voidlessly to encase the foil 23in a right-circular cylinder. This is inserted into a copper extrusioncan, the copper being a necessary part of the final product as explainedabove, and the whole drawn to the final wire size. Let this be Stage B.

ROUTE 2

The Intermediate Member (the array of sixty-one components 10), item 20of FIG. 2, is surrounded by twenty-four filler strips 21 and six cornerfiller strips 22 to present a regular hexagonal exterior. This iswrapped in tantalum foil 23, which acts as a tin diffusion barrier. (Thefiller strips 21 and 22 are described in more detail later.) A thickertantalum can may be expedient in some cases, instead. Let this be StageA. This is extruded down to one-tenth of its starting area. Let that beStage B. The tantalum-clad extrusion-reduced Intermediate Member isinserted into a close-fitting hexagonal copper tube, and seven (ornineteen, thirty-seven, sixty-one . . . ) of the tubes are assembledinto a close-packed hexagonal array. Aluminium arcuate filler strips areapplied to the outside of this array, so profiled as voidlessly toencase the array in a hexagon or right-circular cylinder as convenient;this is inserted into a copper extrusion can and extruded and/or drawnto the final wire size. The copper can may then be removed (bydissolution in nitric acid), and then the aluminium (by dissolution incaustic soda).

ROUTE 3

This is identical to Route 2 except for a modification in case there isno access to a hexagonal-to-hexagonal extrusion press as is necessaryimmediately after Stage A. In Route 3, Stage A is followed by applyingarcuate aluminium filler strips to the outside of this array, soprofiled as voidlessly to encase it in a right circular cylinder, whichis canned in copper. This is subjected to circular→circular extrusion toone-tenth of its starting area. The copper is then removed bydissolution in nitric acid, followed by the aluminium (dissolved incaustic soda). This is Stage B, and Route 2 is rejoined at that point.

ROUTE 4

The Intermediate Member (the array of sixty-one components 10), item 20of FIG. 2, is surrounded by twenty-four filler strips 21 and six cornerfiller strips 22 to present a regular hexagonal exterior. This iswrapped in aluminium foil and then inserted into a hexagonal copperextrusion can, the aluminium serving as a copper-copper antibondinglayer. This is Stage A. The whole is extruded to one-tenth of itsstarting area. This is Stage B. The copper can is dissolved away bydissolution in nitric acid and the aluminium foil is dissolved away bydissolution in caustic soda. The resultant reduced Intermediate Memberhas a space-filling cross section, and seven (or 19 or 37 . . . ) ofthem are voidlessly stacked in hexagonal array. That array is wrapped intantalum foil (to act as a tin diffusion barrier) and arcuate copperfillers are applied round it, so profiled as voidlessly to encase thefoil in a right-circular cylinder. This is inserted into a copperextrusion can, the copper being a necessary part of the final product asexplained above, and the whole drawn to the final wire size.

ROUTE 5

The intention is to assemble seven Intermediate Members 20 in hexagonalarray, as shown in FIG. 3, to form the final superconducting wire. TheseMembers are notionally labelled 20¹, 20² . . . 20⁷, according to theirintended individual positions in the hexagonal array. Member 20¹ is madeinto a regular hexagon by adding filler strips -21 and -22 as explainedin FIG. 4 later. It is wrapped in aluminium foil (to serve as acopper-copper antibonding layer) and inserted into a hexagonal copperextrusion can. Members 20² -20⁷, which are in fact identical, are eachmade into a regular hexagon by adding the filler strips -21 and -22 tothree adjacent sides (those which will abut other Members 20) and addingstrips +21 and +22 to the remaining (open) three sides. The strips -21and -22 have the same shape as their counterparts +21 and +22 but are ofaluminium. (+21 and +22 are identical to 21 and 22 of FIG. 4.) Then theMembers 20² -20⁷ are each wrapped in aluminium foil and inserted into ahexagonal copper extrusion can. This is Stage A. Then all seven Membersare separately extruded to one-tenth of their area. That is Stage B. Thecopper extrusion can is dissolved away using nitric acid, and thealuminium (foil, and strips -21 and -22) is dissolved away using causticsoda. The seven Members 20¹ -20⁷ can now be voidlessly stacked asoriginally envisaged in FIG. 3. It will be observed that they cannot infact be assembled in any other than the correct orientations. That stackis is wrapped in tantalum foil (to act as a tin diffusion barrier) andarcuate copper fillers are applied round it, so profiled as voidlesslyto encase the foil in a right-circular cylinder. This is inserted into acopper extrusion can, the copper being a necessary part of the finalproduct as explained above, and the whole drawn to the final wire size.That final drawing, if started at 77K, allows quite a respectablereduction, such as to 1/10 of area, without exceeding an outputtemperature of 200 C. In that way, the tin (explained in a moment) isnot melted.

At either Stage A or B of any Route, the aluminium pillars 1 aredissolved out using hot sodium hydroxide. Stage A is preferable becausethat dissolution is easier but Stage B is also preferable because theA→B extrusion is easier with aluminium in the pillars 1 than with itsreplacement. The aluminium is replaced by solid tin rods or by moltentin, which is caused to flow into the pillars 1.

The product of each Route may be bench-drawn then taken through wiredies as required, and the wire made into a winding as necessary for anelectrical machine. By this time the individual columns 2 are under tenmicrons across. The product is lightly twisted in use (e.g. 1 turn percm) in order to decouple the filaments electrically.

Then (or at any time after the tin was introduced if there was to be nosubsequent strain greater than about 0.2% within the wire) the wire isheat-treated. The tin in the pillars 1 diffuses through the copper tothe niobium (at no point having any great distance to go), forming insitu Nb₃ Sn (Al5) superconductive kilofilament (but non-touching) wires.

Turning to FIG. 4, a special intermediate is shown enlarged for clarity,made up of hexagonal and part-hexagonal columns, which are identical tothe columns 2 of FIG. 1 in hexagon size, and some of which are furtheridentical in that they contain niobium rods. The composition of eachcolumn is shown. Despite the apparent complexity of the part-hexagonalcolumns, only three different pairs of part-hexagonal dies are neededaltogether. The special intermediate is geometrically the same as thecomponent 10 of FIG. 1 and is extruded in the same way.

Then it is disassembled by etching in hot caustic soda (which removesthe aluminium) to yield the filler strips 21 and 22 (as labelled), whichwere mentioned above.

These filler strips serve to preserve the overall optimum compositionand, by filling the space, allow the subsequent extrusions to beperformed with no distortion of the niobium rods, thereby at the sametime allowing a full use of the volume for conductors (not wasting itwith voids or excess inert material) and minimising the incidence ofadjacent niobium rods touching, which would allow wasteful eddy currentslateral to the rod; in other words our actual rod diameter shouldclosely approximate to the "effective diameter", which in the prior artis wastefully large because of touching rods.

The products of Route 2 or 3 show a network of `veins` of tantalum andpure copper throughout their thickness. Although this is a loss ofpotential superconducting volume, it improves the safety margin if thereis localised heating to above the superconducting temperature, byproviding a nearby `relief pathway` for accepting current and removingexcess heat. The products of Route 1, 4 or 5, on the other hand, havetantalum and pure copper on the outside only, such that--the totalamount of copper being held the same--the volume proportion of tantalumis less for a given barrier thickness. This improves thecurrent-carrying capacity per unit cross-sectional area of the wire butreduces the electrothermal stability of the wire.

We claim:
 1. A cylindrical component for use in fabricatingsuperconducting wire, comprising a central pillar made of at least onematerial selected from the group consisting of a stanniferous,galliferous, germaniferous material, an extrudable removable precursorof the germaniferous material, and aluminiferous material, surrounded bya two-layered array of cupriferous columns, each containing aniobiferous rod, with an outermost layer of said columns beingpolygonal, a cross-section of the component being a plane-fillingshape;wherein the cylindrical component is structured so as to allowstacking of the components, wherein an inner layer and outer layer ofsaid two-layered array have twelve cupriferous columns and eighteencupriferous columns, respectively, and wherein at least some of thecupriferous columns of the inner layer are pentagonal in cross-section.2. A component according to claim 1, wherein the cupriferous columns ofthe outermost layer are equi-sided, equi-angled hexagons incross-section.
 3. A component according to claim 2, wherein at leastsome of the cupriferous columns of the inner layer are identical tothose of the outermost layer.
 4. A component according to any precedingclaim, wherein the central pillar is duodecagonal in cross-section.
 5. Acomponent according to claim 1 further surrounded by removable ductilefiller strips so profiled as to impart to said component a void-freeextrudable cross-section.
 6. A component according to claim 5, whereinsaid extrudable cross-section is one of an equi-sided, equi-angledhexagon and a circle.
 7. An intermediate member comprising aclose-packed assembly of components according to claim 1.